`Struct` represents a structured data value, consisting of fields
which map to dynamically typed values. In some languages, `Struct`
might be supported by a native representation. For example, in
scripting languages like JS a struct is represented as an
object. The details of that representation are described together
with the proto support for the language.

`Value` represents a dynamically typed value which can be either
null, a number, a string, a boolean, a recursive struct value, or a
list of values. A producer of value is expected to set one of that
variants, absence of any variant indicates an error.